


Gaster blaster brothers

by 13thcatofthegate



Category: Swapfell - Fandom, Underfell - Fandom, Undertale, underswap
Genre: M/M
Language: English
Status: Completed
Published: 2017-02-21
Updated: 2017-03-24
Packaged: 2018-09-25 23:53:51
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings, No Archive Warnings Apply, Rape/Non-Con
Chapters: 3
Words: 2,910
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9852614
Author URL: https://archiveofourown.org/users/13thcatofthegate/pseuds/13thcatofthegate
Summary: Tale and Fell Sans as well as Swap and Swapfell Papyrus have turned into their gaster blaster forms and their brothers try to help them turn back to normal.





	1. Tale and Fell

Tale and Fell-

Well this was a surprising turn of events. The last heat of the skeleton bros had left a few of them changed. The Sans of tale and fell and the Papyrus of swap and swapfell had shifted to their gaster blaster forms.

"Oh no, how could this have happened brother? Was I too rough with you when we were in bed?"

"Paps I don't know how this happened but don't worry I'm sure I'll go back to normal." Sans looked back at his worried brother through his now doglike skull. The horns on the top made him look a bit intimidating along with the sharp claws on his hands and feet. A long tail wiped the floor almost hitting the table leg. There was a knock at the door making Papyrus jump. Sans could hear the monsters on the other side and knew right away who it was. Opening the door Edge stood there a red leash in his hand and his brother attached to it. Red stood there in the same state as Sans. "YOUR BROTHER IS LIKE THAT TOO!?" shouted Edge pulling his brother inside the house.

"Edge, you don't need to shout." said Papyrus

"Not like we couldn't hear you coming a mile away anyway." said Sans as Red was let of the leash. As soon as the metal left the collars ring he ran over to Sans almost knocking him over as he gave him a hug. Sans got startled as Red rubbed his muzzle against his in a show of affection like most dogs do. "Oh, uh, he's still in heat if you're wondering." snickered Edge.

"WHAT! You brought him over here like that!" Papyrus almost shouted making Red and Sans jump this time.

"I thought maybe if your Sans had gone through something like this then he'd know what to do." Edge glared at his nicer version but Papyrus didn't back down.

"Hey! Let's not get hostile or someone's going to end up with a lot of teeth to deal with." said Sans making them both stop. "If you came here for help then I'll help Red out. But it's no guarantee that he or I will go back to normal."

Edge gave Sans a smile as if he had won the argument against his brother. Sans let out a growl before heading to the stairs with Red close behind. It was strange watching them climb them on all fours because their feet were longer now. Edge turned to Papyrus as soon as the door was closed to Sans room. "So, what are we fixing for dinner?"

"*Sigh* Alright, we might as well cook something they're going to be hungry when they're done." Papyrus went into the kitchen and started taking things out to cook.

Red seemed to be a bit impatient as he nipped at Sans horns and pulled at his jacket. Now Sans would normally stop Red but he was still in heat as well and his animal instincts were taking over. "R-Red, calm down. We have a few hours to our self, our brothers won't bother us."Red let out a bit of a whimper making Sans feel bad.

"Boss didn't help me this time." he whined, his tail curled up between his legs.

Sans was really sorry now Red must be in pain from not having any help through his heat. Sans rubbed his skull against Reds making his copy push against him. Sans started licking at the side of Red's jaw. Red let out another whimper as he was led over to the bed and sat down. Sans helped him take off his jacket and found out he didn't have on his shirt. He could see Red's soul pulsing in his ribcage giving his bones a pinkish tinge.

"Red, are you okay with this?" There was a strange look on his face as he grabbed at the top of his shorts and yanked them off catching it a bit on his feet. Red laid back on the bed naked waiting for Sans to join him. Shrugging his shoulders Sans took off his own clothes and crawled onto the bed. It was strange with their new forms, kissing was out of the question but they could still lick at each other. Every once in a while a tail would get caught under a leg or foot until Sans wrapped his tail around Red's to keep it out of the way. Red didn't even mind that Sans was leaving claw and bite marks when he ran his hands down his ribs and bit at his collar bone. "S-Sans..."

"Okay, Red." Sans coaxed him to turn on to his stomach and moved his hips into the air. Red's magic had formed into the male parts living Sans a bit disappointed. Leaning over Red's back he nipped at the back of his neck and moved to his shoulder. Letting out a low growl he grabbed Reds pelvis making him yelp. Sans tried to be as gentle as he could but those animal instincts were taking over. "Fuck, Red." Sans growled as he slowly started to press into him. Red started whining as Sans started to move, his clawed hands dug into the bed leaving large gashes. Sans reached for Red's dick rubbing it in time with his thrusts.

"S-Sans... AH! Bite me some more, AHH-!!" Sans did as asked starting with his horns and moving to his jaw leaving large red teeth marks. When he reached his ribs he managed to place a few or them into his mouth an gnaw on them. Red screamed from the mix of pain and pleasure making Sans move a little faster. Letting go of Reds ribs he moved to his soul that was fluttering in his chest. "Red... are you okay?"

*Gasp* "Ah!- yes... keep going... guh- I'm a-almost there." Red suddenly held his breath as something brushed his soul. When it happed again he let out a shuttering whine. Sans was licking between his ribs to reach the red glowing heart. It was too much, Red whined so loud as he came Sans was sure their brothers must of heard. Sans came soon after adding to the mess that Red had created. Falling to the bed Sans noticed that their bodies had started to change back a bit. Red had passed out but his skull had turned back to normal, the claws and feet were shorter as well. Wrapping his arms around Red he let out a sigh and drifted to sleep.

A moment later there was a loud knock on the bedroom door. "Sans? Are you and Red okay? We haven't heard anything for a while." It was Papyrus and Sans had to clear his throat before he could speak.

"Ahem- Ye-Yea Pap, we're fine." Sans said reaching up and finding his own skull back to normal.

"Well supper is ready, Edge has been helpful but I don't know how much longer that will last."

"We'll be down in a bit..." Sans carefully woke Red and they got dressed. When they made it to the kitchen Edge looked happy to see that they looked back to normal. Well he thought they were back to normal. They still had longer foot bones and claws. When both Sans and Red sat down they still had long tailbones and they wagged when they got happy.


	2. Swapfell brothers

**Summary for the Chapter:**

> Slim in in blaster mode and his brother lets his guard down.

Swapfell-

Raspberry sat in his brothers lap with his arms wrapped around his much larger chest. Slim in return had his arms wrapped around him in a light hug keeping his lord as close as he wanted. Raz let out a sigh felling a hole hell of a lot safer now that his brother was in gaster form again. He had done this a few times before when he was younger and every time he was kept safe and sound from any threat. When his brother was like this he could relax and didn't need to worry about anything.

Slim looked down at him nudging his snout against his skull. Reaching up Raz guided his head to where he could click his teeth against the sharp gold fang. There was a bit of a purring sound that was his reward making him rest back against his chest. "My lord... are you okay?"

Raspberry hugged him tighter. "Hmm- yea, I'm fine."

"It's been a long time since I seen you this relaxed." Raz looked up at the dark orange glowing eyes. "Well... I know I'm going to be safe when you're like this."

"Safe?" Slim moved his clawed hand to his lords lower spine getting a blue blush to cross his face. "Safe from what?"

"...F-From danger... Fr-From attacks." Those claws carefully ran up and down his spine sending little shivers through his bones. "What about me?"

There was a little gasp as Slim reached under Raspberry's shirt brushing his ribs. "I-I don't need to be a-afraid of y-you."

"Really... you should be." Raz was about to ask what he meant by that but quickly sharp teeth headed for his neck and a playful growl escaped Slims maw. There was a moment of fear as those teeth bit down but not hard. It felt almost like he was being strangled a bit making him reach up and try to get lose by forcing the jaws open. "P-Papyrus..."

Slim lifted his head picking Raz up from his lap. This was so embarrassing he looked like a chew toy as he was dragged up the steps to the bedroom. Luckily Slim had used one arm to keep him from hitting the steps. Once they got to the room Slim placed his brother on the bed finally letting go of his neck.

*cough* "Don't EVER do that again!!" His warning seemed to go unheeded as his shirt was roughly removed followed by his pants. When Raz tried to correct his position to a more comfortable one he was quickly pined down. "What is wrong with you?!" He shouted.

Slim looked down at his brother liking the scared expression on his face. "Sans, do you love me?" That look of fear quickly turned to confusion as Slims grip loosened.

"...Wha-What? What the fuck are you talking about?"

"Do you love me?" he repeated again almost growling. "...You are a stupid mutt, w-why would I love you?" Slims grip got tight again as he dragged Raspberry to the middle of the bed and pined him beneath him laying between his legs.

"I think you're ling." Raz tried to get lose but all it did was make Slim grind against his pelvis. The rubbing stirred up his magic making a blue tinge appear across his face.

"P-Papyrus..." Slim seemed to get a big grin on his face as he continued. "Papyrus, s-stop...." Raz was starting to gasp as his magic took form. Slim looked down seeing a blue mound between his legs. Letting go of Razs arms he moved to where his muzzle brushed against it. He could hear a muffled gasp as his tongue slid between the folds and across the little nub hidden between them. "P-Papyrus, AH!" Raz tried to move away but those long arms grabbed him again keeping him in place. Slim kept licking at the entrance until he suddenly stopped looking up at Razs flustered face. "Wh-Why did you stop?" he gasped looking back at him.

"Do you love me?" Slim asked again.

"..." Raz turned his head to not look into those orange eyes. *growl* "Suit your self my lord." His snout dipped back down to continue forcing another muffled moan. Raz was trying to keep his brother from knowing how good this was feeling when he felt something warm and wet press into him. Gasping he grabbed at Slim feeling fangs scrape at his eto-flesh. Slim was using his tongue to go deep inside.

"Papyrus! AH, please, AHHHAAA-" Slim pulled away licking the light blue juices from his teeth.

"Heh, my lord that tasted amazing." Raz tried to catch his breath as his body still shook. Crawling up from between his legs Slim wrapped his arms around him cradling Raz against him. "...I changed my mind... I do love you."

Raz buried his face into his brothers chest. For a moment He started to relax when he realized something. "What about you?"

"...Don't worry about it, I'm sure we can take care of it latter."


	3. Underswap

**Summary for the Chapter:**

> Swap Pap and Blue have been trying to get together for a while but stuff just keep happening and things get in the way.

Underswap

 

Blue carefully went to his brothers bedroom door hoping that his brother had returned to normal. Knocking he heard a growl on the other side making him jump back.

"Bro?" He could hear Stretch weakly ask.

"Brother, you've locked yourself in there for two days now... I'm starting to worry..." There was a sound of something large shuffling in the room before it stopped and the door was unlocked. Blue reluctantly reached for the nob to push it open seeing the dimly lit room. The mess in his brothers room was no worse than normal from the looks. The curtains had been pulled shut leaving only a small sliver of light. Blue walked in kicking a couple empty honey containers that skittered across the floor into the dark.

"B-Brother?" Blue was getting worried not seeing Stretch in the room. There was a noise as the door closed making him turn around and be taken off his feet by a very large version of his brother. Blue gasped as he was quickly taken over to the bed and dropped onto the covers that were balled up as if his brother was having a hard time sleeping. The monster that had picked him up sat down across from him.

"I hate it when you do that!" Stretch looked away from his brother the long skull allowing him to use one eye to still look at him.

"... You're still in the blaster form..." Stretch sat there his legs tucked under him and wearing only his khaki shorts. His long tail swished resting on one side of blue wrapping around the balled up cover he was sitting on.

"Sans... I need some help..." Blue gulped seeing his orange eyes turn back to look at him.

"I-I'll call Red or Sans..." He was about to get up when the tail tightened around the cover scooting him closer. Stretch leaned over him keeping him from running away.

"Why ask them, you're all I need."

Blue yelped as he was pushed into the bed and pinned down. Looking up at his brother her was afraid at what might happen next. "P-Papy... What are you doing?" Stretch reached for his brothers boots and yanked them from his feet. Next was his armor leaving him in only in a white undershirt and gray pants.

"Papy... please don't do what ever is on your mind..." Stretch paused and leaned back over top of him. "What do you think is on my mind?"

"....Uh, well... don't... don't bite me. I think those teeth are going to hurt..." Blue flinched when Stretch grabbed him hugging the small skeleton and giving him a deep laugh that sounded like a growl. Blue relaxed a bit laughing along with his brother.

"Don't worry, I'll try and not bite you." Blue quickly stopped laughing when Stretch started nudging at the hem of his shirt with his snout. Once under the shirt Blue could feel something wet run up his spine. Letting out a gasp it happened again going from the top of this pelvis to the bottom of his ribs in one stroke. "P-Pappy... Ah! W-What are y-you doing?"

All blue could see was those orange eyes looking up at him. After a few moments Blue's hands were let go as the large clawed fingers moved to grab his pants and pull them down a bit to allow the licking to go lower.

"Ah! Please... s-stop..." Stretch pulled his head away as blue magic formed a belly and lower parts.

"Heh, you're so cute when your tummy appears." A clawed finger brushed Blues side making him shiver and flinch. "Th-that tickles..." Looking at his brother Blue noticed an orange glow coming from his pelvis. "...B-brother?" Stretch reached for the grey sweat pants that were half way down Blues hips. Blue quickly grabbed the clawed hands trying to keep him from pulling them down. Blue couldn't stop him, in his current state Stretch was way stronger than normal and removed the clothing. Scared Blue tried to get away but that tail stopped him throwing him back to the bed. Blue tried to get back up after his face plant into the covers and pillow but a sudden weight pined him down.

"This would be so much easier if you just help me out Blue..." There was a sob that came from the smaller body that was pined to the bed. "N-not like this... I don't want it like this..."

Stretch froze hearing him sob into the pillow. Blue shut his eyes tight when he felt Stretch move but he moved away from him. Once the pressure of the bigger body was gone Blue opened his eyes. Stretch was sitting on the other end of the bed his tail wrapped around his legs and his head low. "I'm sorry... maybe it's best you call one of the others..."

Blue wiped away the tears in the corner of his eyes. "I... just don't want our... first time to be like... that."

Stretch chuckled and looked over at him. "Don't worry, maybe next time."

**Author's Note:**

> I'm going to try and make this a series and draw them into comics soo, hope to have the next story here soon.


End file.
